1. Field of the Invention
This invention relates to an optical reproducing device and more particularly, is suitable for use in a optical disc player wherein a laser beam is used to read information, such as audio or video signals, recorded on a track of a disc in the form of pits.
2. Description of the Prior Art
In an optical reproducing device such as an audio or video disc player, a light beam is used to read an information signal carried by a rotating record carrier in which pits are formed on a track of the record carrier.
To ensure that the reading light beam is always directed on a track during the reading of the carrier, the focusing spot of the beam must be adjustable in an axial direction so that it can follow the track. It is to be appreciated that in general, the moving carrier is not perfectly flat.
The focusing point of the laser spot along the optical axis of the beam may be adjusted in several ways. One way is to move the reading lens or focusing lens along its axis by an electromagnetic motor moving perpendicularly to the plane of the carrier.
The electromagnetic motor can be controlled by a focusing control system which includes a detecting means for detecting the focusing deviation or error of the reading beam. The detecting means, in general, detects the focusing error on the basis of reproduced signals from the track. Since the dynamic ratio of the detecting means is usually very low, the focusing control system has a very narrow capture range, for example, about .+-.10 .mu.m. Outside this range it is impossible to detect a focus error, yet the focus control system is not properly locked.
At the beginning of a reproduction operation, therefore, it is necessary initially to bring the laser spot close to the focussed position so that the control system remains stable during subsequent operation.
Lens movement can also be performed by another control system which always moves the lens in the same direction from a rest position until it is positioned within the capture range or pickup zone of the focusing control system. The control system or capture range search system has a speed control loop for moving the lens at a constant speed, because, if the speed is too high, the system will evershoot the pickup zone.
The speed control loop includes a speed sensor for detecting the speed of the lens driven by the electromagnetic motor. When the motor has a moving-coil arrangement, such as a dynamic loudspeaker, a counter electromotive voltage induced across the moving-coil may be representative of the moving speed of the lens. It is, however, difficult to represent the moving speed accurately, as the moving-coil has resistances which are affected by temperature variations. Such a conventional system, therefore, has disadvantages since the moving speed can not be maintained constant, resulting in mis-operation of the capture range search system. Also, when the speed is too high, the system may pass beyond the capture range. When the speed is too low, the system can take too much time to reach the capture range.